There is a widespread need for images of subsurface conditions and materials in connection with various industrial applications. Such imaging can be used to map subsurface water movement or mineral deposits. It may also be sued to assist in the recovery of oil or gas, or in environmental remediation efforts.
Electrical Impedance Tomography (“EIT”) has been used to provide subsurface imagery between pairs of boreholes for mapping hydraulic conductivity, soils resistivity, mineral deposits, vadose zone water movement, as well as for various medical imagery applications. The technique is generally based on automated measurement and computerized analysis of electrical resistivity changes caused by natural or man-made processes. It is particularly powerful for measuring temporal changes in flow characteristics. To determine resistivity, current is passed through a material such as earth from one electrode pair to another electrode pair either in an adjacent well or in the same well. By making electric potential measurements with both the source and receiver at a number of different positions in their respective boreholes, one can obtain data for a multitude of current paths that cross the region to be imaged.
EIT and its current applications present certain disadvantages, however. For example, temperature information has not been previously considered in such a process. These methods do not take into account or take advantage of the fact that as resistivity changes with heating, the actual temperature of the fluids and other subsurface materials can be determined from such information. Furthermore, the quality and utility of images created using current systems and methods is limited. Conventional cross-borehole EIT methods do not give a three dimensional image over a large volume. They simply image resistivity between boreholes with no complete mapping of the directionality or spreading of fluids with temperature measurement of the heated fluids.
The proposed EMIT system and method in this invention presents an alternative for subsurface imaging. The EMIT system disclosed herein measures the temperatures of the fluids and other subsurface materials of use and analysis during industrial applications. Furthermore, the disclosed system with surface arrays allows for measurements over a full 360 degrees in azimuth from the surface to the desired depth. Thus, the system allows three dimensional images over a large volume.